Alloreactivity is initiated by T cells that specifically recognize mismatched major (MHC) and/or minor histocompatibility antigens (MiHA). Graft-versus-host-disease (GVHD) is a potentially lethal consequence of bone marrow transplantation (BMT) in which alloreactive donor T cells undergo robust expansion and functional differentiation within recipients and can cause severe damage to the gut, liver, lung and skin. Therapeutic immunosuppressive regimens that prevent T cell activation can limit the deleterious effects of GVHD. However, because commonly used agents are broadly immunosuppressive, they also render recipients susceptible to life threatening infections. When used as immunotherapy for hematopoietic malignances (e.g. leukemia), the therapeutic potential of allogeneic BMT relies on the graft-versus-leukemia (GVL) effect to eradicate residual tumor cells through immunologic mechanisms. Thus, identification of targets important for alloreactivity but not GVL or responses against infectious agents may lead to development of novel approaches for preventing GVHD, and also allow more widespread use of BMT in a variety of malignant or non-malignant hematopoietic disorders. We have found an essential requirement for PKC?, a key T cell-specific PKC isoform, in alloreactivity and GVHD induction. Importantly, PKC?-/- T cells retain both the ability to respond to virus infection and induce GVL post-BMT. These findings indicate that PKC? is a potentially unique therapeutic target for the prevention of GVHD while preserving GVL effect and protective responses against infectious agents. The goal of studies proposed here is to define key aspects of the molecular and cellular function of PKC? in T cell alloreactivity, and determine whether PKC? is a viable therapeutic target for inhibition of alloreactivity and GVHD. We will accomplish this with the following three specific aims: Aim 1: Defining PKC?-dependent and independent mechanisms in alloreactivity. Aim 2: The role of PKC? in CD4 effector and regulatory T cells, and in T cell migration and survival. Aim 3: The role of PKC? in the beneficial effects of donor T cells after BMT.